Main Defects in Focused Ion Beam (FIB) Processing

Focused Ion Beam (FIB) is a micro- and nanofabrication technique, the basic principle of which is similar to that of Scanning Electron Microscopy (SEM), which employs a beam of ions emitted by an ion source accelerated and focussed to act as an incident beam, and the process of collision of high-energy ions with atoms on the solid surface can sputter and strip off the atoms from the solids, and thus FIB is more often used as a tool for the direct processing of micro- and nano-structures. In combination with a gas injection system (GIS), the FIB can assist in chemical vapour deposition, localisation-induced deposition growth of micro- and nanostructures, or selective enhancement etching of specific materials and structures. In the practical application of focused ion beam processing to fabricate micro and nano structures, due to the characteristics of the FIB itself and the material being processed, the final processed and fabricated structures sometimes produce defects, which mainly include: tilted sidewalls within the focused beam spot, the ions show Gaussian distribution characteristics, and the closer to the centre of the beam spot, the larger the relative number of ions. If the ion beam is bombarding the sample by etching at individual pixel points, holes with conical cross-sectional profiles will form. As the etching depth increases, the taper of the cross-section will gradually decrease until saturation. Depending on the material and its crystal orientation, the cross section will typically have a taper of 1.5 to 4°. To obtain a cross-section that is perfectly perpendicular to the sample surface, it is common to artificially tilt the sample at a specific angle to compensate for the deviation of the cross-section from the angle of incidence of the ion beam. Alternatively, cutting by lateral incidence can be used to flexibly process 3D micro- and nanostructures with more complex shapes by defining an etching pattern to control the angle between the cross section and the surface. Curtain structuresAnother issue of concern when processing sample cross sections with focused ion beams is the flatness of the cross section, which sometimes results in vertical streaks in the cross section, known as curtain structures. The formation of curtain structures is closely related to the inherently sloping sidewalls of the focused ion beam cut, and when there are topographic undulations or compositional differences on the surface of the sample, differences in the etching rate are produced, and curtain structures are formed. For the curtain structure caused by surface topographic relief, the solution is usually to grow a protective layer on the sample surface by FIB-assisted chemical vapour deposition to flatten the surface, or by changing the direction of incidence of the ion beam, starting from the undulating surface to cut, so as to avoid its effect. For curtain structures caused by compositional differences, they can be eliminated by wobbling the cut so that the ion beam is incident at multiple angles. Non-uniform etching focused ion beam can be directly and rapidly processed to produce micro and nano planar graphic structures, for amorphous materials or monolithic single-crystal materials, FIB etching usually results in very flat wheel-over shapes and bottom surfaces, but for polycrystalline and multicompound materials, due to the different orientations of the individual grains, the etching rate will be different in the different grain regions, and will often show non-uniform etching, and the bottom surfaces are not The bottom surface is not flat. The non-uniform processing defects of polycrystalline material etching can be improved by increasing the residence time of each point of the ion beam scanning. When a focused ion beam bombards a solid material, some of the atoms of the solid material are sputtered out of the process, and some of the atoms fall back to the surface of the sample in a process called redeposition. By increasing the residence time of the ion beam at each point, the effect of redeposition is enhanced, and the redeposited atoms have a higher chance of falling into depressions, which can act as a flattening effect, thus improving the flatness of the etched substrate.

The left figure shows the cross-section running water effect after ion beam cutting when XeF2 is not used, and the right figure uses XeF2 assisted etching, the surface is flatter after cutting For non-uniform etching defects produced by polycompound materials, gas-assisted enhancement of the etching can usually be used to make slower escaping atoms and the reactive gas to form a compound with lower melting point and be removed by rapid etching. Reaction Gas Residual Contamination Focused Ion Beam Processing combined with a gas injection system can enable assisted chemical vapour deposition to locate and grow specific nanostructures, a method known as focused ion beam induced deposition. However, residual contamination by reaction gases is a problem that cannot be ignored, and also reaction gases may remain on the sample surface causing contamination. Removal of residual reaction gas contamination is usually done by heating the sample to make it desorb faster, and can also be removed by etching using ion bombardment. Conclusion Focused ion beam technology has gradually been applied in many fields due to its direct and flexible advantages. In the process of processing and fabricating various micro- and nanostructures, processing defects are sometimes generated, and analysing the physical root causes of these processing defects and investigating the methods to mitigate or eliminate these defects can improve the processing performance of the focused ion beam, and try to obtain micro- and nanostructures that conform to the expected design.